Many ground-based systems require knowledge of the attitude (true-heading, pitch and roll) in which the elements are positioned relative to earth. In order to obtain accurate attitude information it is necessary to find the true North. It is well known in the art to use gyrocompasses and staged gyros to find the North by measuring the direction of Earth's axis of rotation.
North-seeking devices are typically a compass consisting of a motor-operated gyroscope whose rotating axis, kept in a horizontal plane, takes a position (attitude) parallel to the axis of the earth's rotation and thus points to the geographical north pole instead of to the magnetic pole. Illustrative examples of such systems, which also describe details of operation of conventional systems can be found, for instance, in U.S. Pat. No. 5,272,922 and in U.S. Pat. No. 7,412,775, the description of which is incorporated herein by reference. The principle described in said patents can be referred to as a mechanical gyrocompassing system.
Mechanical gyro compassing systems and apparatus present two main disadvantages, inasmuch as they require long calibration times of the order of minutes, before the North can be found. This is due to the fact that the rate of change of the gyroscope's angular momentum vector equals the applied torque, and therefore a gyroscope having a large angular momentum vector influenced by a comparably small torque requires significant time to align the angular momentum vector with the axis of rotation producing the torque. Secondly, because of the high precision required, sophisticated and extremely expensive gyroscopes must be used.
Another well established and widely used methodology for north finding relies on the use of one or more rate gyros mounted on a stage/indexing table. The stage/indexing table is positioned on a leveled platform (leveling can be computed by using accelerometers). The rate gyros sense the component of the earth rotation vector in several directions (using stage/indexing) and the north direction is derived, using numerical computations. This methodology requires static conditions for the platform during the process of north finding, and normally takes several minutes before a converged solution is achieved. This method is usually referred to as a “staged gyros north finding”.
Generally speaking, staged-gyros north finding achieves a given accuracy faster than the older technology of mechanical gyro compassing. A profound limitation of the north finding technique, however, is the need for static conditions during operation. Even small changes in the attitude of the platform cause the need for recalculation of the north direction.
As will be easily appreciated, systems which require frequent finding of the North in quick succession are severely hampered by the need to allow for long calibration times. For instance, artillery may need precise directional knowledge, between consecutive firings
It is an object of the present invention to provide an improved system that allows using North data without requiring long calibration periods after each dynamic movement. The present invention improves the capabilities and performance of north finders based on the staged gyros north finding technique. Fast reaction to dynamics is a key feature of Inertial Navigation Systems (INS), but the cost of these systems is high. The present invention bridges the gap between a conventional North Finder and an expensive INS, by offering a high accuracy attitude solution with a fast reaction to dynamics and at a relatively low cost.
It is another object of the invention to provide a system that overcomes the aforementioned drawbacks of the prior art, which is relatively inexpensive and robust.
Other objects and advantages of the invention will become apparent as the description proceeds.